FIG. 6 illustrates a conventional permanent magnet rotor. Permanent magnets 11 of the rotor are fixed to a rotor core 12 by an over mould member 10. The magnets 11 are arcuate in shape, also known as segment magnets. The middle portion of each magnet 11 is thicker than the circumferential ends of the magnet such that the portion 13 of the over mould member 10 covering the middle portion of the magnet 11 is thinner than the portions 14 of the over mould member 10 covering the circumferential ends of the magnet 11. The reliability of the rotor is relatively low as the thinner portion 13 may be broken during rotation of the rotor.